Low frequency amplifier



'Aug 15, 1939. CARL-ERIK GRANQVIST 2,169,837

LOW FREQUENCY AMPLIFIER Filed March 29, 1938 INVE TOR CAR "ERIK GRAN 51'ATTORNEY Patented Aug. 15, 1939 UNITED STATES LOW FREQUENCY AMPLIFIERCarl-Erik Granqvist, Stockholm, Sweden,

or to Hazeltine Corporation,

Delaware Application March 29,

assigna corporation of 1938, Serial N0. 198,625

In Sweden September 3, 1937 '7 Claims.

This invention relates to low-frequency amplifiers and. particularly toaudio-frequency amplifiers stable in operation in modulated-carriedsignal receivers.

In modulated-carrier signal receivers, and particularly insuperheterodyne receivers for shortwave reception, it is frequentlydifficult to obtain stable operation. A superheterodyne receiver, tunedto a station in the shortwave por- 10 tion of the frequency spectrum,may go into self-oscillation if there is a certain amount of detuning. Afeedback of energy from the audiofrequency portion of the receiver tothe local oscillator may cause the frequency of the local oscillator tovary about its normal frequency. If the receiver is correctly tuned,this variation of the local oscillator frequency results in a reductionof the input to the audio-frequency channel of the receiver for eitheran increase or a decrease in the frequency of the local oscillator fromits normal value. Thus, the resulting frequency which is applied to theaudio-frequency channel is twice that which causes the oscillatorfrequency to vary, and self-oscillation of the receiver is notsustained. However, with any mistuning, a variation in the intermediatefrequency about its mean value results in a decrease in theaudio-frequency input voltage for one direction of the frequency shiftsand an increase for the other. The receiver may then go intoself-oscillation. This form of instability manifests itself as a veryannoying low-frequency tone, consisting of about 10 cycles per secondfor the signal type of receiver, and may easily destroy the usefulnessof its received program. The abovementioned phenomenon may also occur inmodulated-carrier signal receivers provided with automatic frequencycontrol. The carrier Wave of the station being received may become very40 weak so that the automatically operated frequency control does notfunction properly. When thereafter the carrier wave increases instrength and the automatic frequency control system becomes effective, acertain detuning may exist in the receiver circuits before the tuning iscorrected by the automatic frequency control system and this detuningmay be sufficient to start the periodically recurring blasts of sound.

The primary cause of these annoying low-fre quency tones or sound blastsis that the impedance of the source of space current is not sufiicientlysmall at the low frequencies at which the phenomena occur. Since theimpedance of the source of space current is, in practically allreceivers, larger at low frequencies than at higher ones, theamplification of low-frequency currents in the audio-frequency portionof the receiver introduces a low-frequency voltage component in theanode voltage supplied to preceding tubes of the receiver. As theimpedance of the source of space current increases with decreasingfrequency, this reaction increases. The amplification of theaudio-frequency amplifiers is, however, simultaneously reduced, so thatthe reaction passes through a maximum at some low frequency in theaudio-frequency range. If a component of the alternating voltageintroduced in the anode voltage supply at this frequency is in phasewith the input voltage to the audio-frequency portion of the receiver,the operation may become unstable and self-oscillation may occur. Ifthis component of voltage is out of phase with the input voltage to theaudio-frequency portion of the receiver, the resultant input voltage tothe audio-frequency portion of the receiver is reduced at thisfrequency, causing distortion and unsatisfactory operation. By means ofcarefully designed filters in the space current circuits of the severaltubes of the receiver, it is possible to eliminate the above-mentioneddisturbances, but the elements for such filters are bulky and expensive.It has also been proposed, in condenser-resistance coupledaudio-frequency amplifiers, to reduce the size of the couplingcondensers between the several stages. This method is not practical inall cases, however, for the reason that such a change causes a.considerable reduction in amplification at the lower frequencies,resulting in objectionable frequency distortion within theaudio-frequency range.

It is an object of the present invention, therefore, to provide animproved low-frequency amplifier which is stable in its operation.

It is a further object of the invention to provide an improvedaudio-frequency amplifier for a superheterodyne receiver which is stableunder all operating conditions.

In accordance with the present invention, a low-frequency amplifier isdesigned so that alternating components of voltage in the space currentsupply, caused by alternating space currents of one or more of theamplifier tubes, are such as to cause minimum reaction on the input tothe amplifier through preceding tubes coupled to the amplifier.Specifically, in accordance with the invention the amplifier is designedso that the alternating current components of the space current supplyare displaced 90 degrees from the input voltage to the low-frequencyportion of the system at the frequency at which the receiver reaction isgreatest.

In the preferred embodiment of the invention, the receiver is of thesuperheterodyne type and comprises two stages of condenser-resistancecoupled audio-frequency amplification. The constants of thelow-frequency portion of the receiver and the power supply, or both, areso proportioned that a phase displacement is introduced in thealternating component of the anode voltage at the frequency of greatestreaction, the

alternating anode voltage component being displaced 90 degrees from thesignal input to the audio-frequency channel of the receiver. Thehigh-frequency portions of the receiver cause practically nodisplacement at this low frequency except the ordinary phase reversalsin the preceding tubes.

For a better understanding of this invention, together with other andfurther objects thereof, reference is had to the following descriptiontaken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

In the accompanying drawing, Fig. l is a circuit diagram, partlyschematic, of a complete superheterodyne receiver embodying the presentinvention; Fig. 2 is an equivalent circuit diagram of the source ofspace current supply of Fig. l at low frequencies; Fig. 3 is a vectordiagram illustrating the phase relationship of voltages at variouspoints in the audio-frequency amplifier of the receiver; and Figs. 4 and5 are graphs showing certain operating characteristics of theaudiofrequency portion of the receiver.

Referring now more particularly to Fig. 1, there is shown schematicallya complete superheterodyne receiver of a conventional design embodyingthe present invention in a preferred form. In general, the receiverincludes a radio-frequency amplifier III, having its input circuitconnected to an antenna II and ground I2 .and its output connected to afrequency changer or oscillator-modulator I3. Connected in cascade withoscillator-modulator I3, in the order named, are anintermediate-frequency amplifier I4 of one or more stages, a detectorand A. V. C. supply I5, an audio-frequency amplifier I6 of two stages,and a sound reproducer IT. A bias derived from A. V. C. supply I5 issupplied over conductor I5 to one or more stages of amplifier II],oscillatormodulator I3, and one or more stages of intermediate-frequencyamplifier I4 to maintain the input to detector I5 within a narrow rangefor a wide range of received signal intensities.

It will be understood that the various circuits just described, otherthan the audio-frequency amplifier I6, may be of conventionalconstruction and operation, the details of which are well known in theart, rendering further description thereof unnecessary. Consideringbriefiy the operation of the receiver as a whole and neglecting for themoment the arrangement of this invention presently to be described, adesired received signal is selected and amplified by radio-frequencyamplifier I0, converted to a modulated intermediatefrequency carrier inoscillator-modulator I3, amplified and selected byintermediate-frequency amplifier I4, and rectified by detector I5,thereby deriving the audio frequencies of modulation. The audiofrequencies of moduation are, in turn, amplified by audio-frequencyamplifier I6 and reproduced by sound reproducer II. The A. V. C. biasderived from source I5 and applied to radiofrequency amplifier I8,oscillator-modulator I3, and intermediate-frequency amplifier I4maintains the input to detector I5 within a relatively narrow range fora wide range of received signal intensities.

Referring now more particularly to the parts of the system involving thepresent invention, there is provided in Fig. 1 a two-stageaudiofrequency amplifier including tubes I8 and I8. Audio-frequencysignal voltages are supplied from the output terminals A and B ofdetector I5 to the input circuit of tube I8 through the couplingcondenser I9 and bias resistor 28. The output circuit of tube I8 iscoupled to the input circuit of tube I8 through the coupling condenser2! and bias resistor 22, its output circuit, in turn, being coupledthrough an audio-frequency transformer 23 to sound reproducer II. Asource of space current supply is provided for the receiver comprising arectifier tube 24 supplied from a convenient source of alternatingcurrent through a transformer 25. A filter circuit comprising a seriesinductance element 26 and shunt condensers 21 and 28 is interposedbetween the output circuit of rectifier 2 1 and the space currentcircuits of tube I8, including transformer 23; of tube I8, includingseries-connected resistors 29 and 38 having their common terminalgrounded for high-frequency currents through a condenser 3|; and of thepreceding tubes of the receiver, supplied from the terminal C. Screenand grid-bias potentials are supplied to the tubes of the receiver in aconventional manner.

In considering the operation of the circuit just described, it will beseen that the alternating space current of tube l8 passes through thefilter 2B2'I-28, building up alternating voltages thereacross which aregreater at the lower audio frequencies. In order to avoid interferingalternating voltages of low frequency in the anode voltage supply of theseveral tubes of the receiver, the filter 262'I-28 and the rectifier 24should possess a very small impedance to currents of these frequencies.The impedance of this filter is, however, considerable at lowfrequencies, which results in objectionable variations in theanode-voltage supply. At such low frequencies, the impedance of thespace current supply shown is approximately the equivalent of acondenser 32 in parallel with a resistor 33, as shown in Fig. 2, thereactance of element 26 being negligible at these low frequencies.Therefore, condensers 2'! and 28 can be replaced by a single equivalentcondenser 32. The impedance of rectifier 24 and of the secondary windingof transformer 25 is practically a pure resistance at these lowfrequencies and may, therefore, be represented by resistor 33.

The space current of tube I8 is filtered through resistance 29 andcondenser 3i, the anode resistor 39 having a resistance considerablysmaller than the resistor 22 so that the alternating voltage developedacross resistor 38 is substantially opposite in phase to the alternatinggrid voltage applied to tube I8. The relation of the alternatingvoltages in amplifier I6 is shown in Fig. 3. In this analysis it isassumed that the filter 29, 3| is effective substantially to suppressalternating voltages from the anode-voltage supply, The output voltageof detector I5, that is, the audiofrequency voltage between theterminals A, B, is indicated by the vector 61, this voltage beingresolved into two components e10, representing the voltage drop acrosscondenser I9, and em, representing the voltage drop across resistor 20.The voltage cm is applied to and amplified by the tube I8, the outputvoltage of this tube being 180 degrees out of phase with respect to em,although for the sake of simplicity it is indicated in Fig. 3 by vectorez in phase with em but with a negative sign. The vector 62 is resolvedinto two components, -e2c, representing the voltage drop acrosscondenser 2|, and c2a, representing the Voltage drop across the resistor22. The voltage -2R is applied to and amplified by the tube I8 to thevoltage 63 in Fig. 3. Owing to the fact that the anode circuit of tubeI8 does not consist of a pure resistance, the alternating anode voltageex is not in phase with the space current of the tube and consequentlyis not in phase with the voltage e2R. Since the operation of the audioamplifier is being analyzed for low frequencies, the inductance in theoutput circuit of tube I8 can be disregarded However, the capacitivereactance of the filter circuit 252i28 is effective to produce theabove-mentioned phase shift of the vector es. The anode circuit of thepreceding tube it also effectively adds resistance to the output circuitof tube l8. This resistance, however, may be disregarded inasmuch as itis considerably larger than the resistance of the source of directcurrent supply. The anode voltage 63 is retarded in phase somewhatrelative to the voltage "62R. The voltage as may then be resolved intotwo components en, the voltage developed across transformer 23, and ez,the voltage developed across the filter 25-2l28, The voltage ez isalways located within the quadrant indicated in Fig. 3 by the angle aand its phase relationship in this quad-- rant for a given frequency isdetermined by the size of the equivalent elementts 32 and 33 of Fig. 2.The voltage ez appears between the point C in the anode supply conductorfor preceding poitions of the receiver and ground; that is, betweenpoints A and C. Inasmuch as the voltage 62., after being fed back topreceding portions of the receiver, again appears across the terminalsA, B, either with no shift in phase or dislaced 180 degrees by thepreceding tubes of the receiver,

it is the component of this voltage in phase with the voltage 61 whichcauses the receiver to go into oscillation, As the magnitude of thevoltage which is fed back varies under different operating conditions,instability may easily occur when it exceeds a critical value. This isespecially true in superheterodyne receivers for short-wave re ception,but oscillation will also occur in other kinds of low-frequencyamplifiers.

As a rule, the frequency at which this selfoscillation occurs is verylow. It is not sufiicient to provide such a phase shift of this voltagethat it is fed back to the points A, B with a component in phaseopposition to the voltage 81 so that there is obtained a negativereaction which prevents oscillation. Under these conditions, in asuperheterodyne receiver having even a small amount of mistuning, thefrequency modulation in the oscillator causes an amplitude modulation tobe fed back to the detector, which, if in phase opposition to thedetector input, will cause distortion. Therefore, in accordance with thepresent invention, the voltage ez which is fed back topreceding portionsof the receiver is caused to be displaced degrees in relation to thesignal voltage (21 impressed on the audio-frequency amplifier it at thefrequency which causes maximum disturbance in the receiver.

In order to explain more clearly the'fun'ction and importance of acorrect dimnsioning of the phase-displacing elements, the impedance Z ofthe source of space current supply, the audio-irequency amplificationcharacteristic k0 of a conventional receiver without low-frequencyreaction of the above-mentioned type, and the audiofrequencyamplification kr of the receiver with low-frequency reaction of theabove-mentioned type are shown in Fig. 4 as a function of frequency Whenthe impedance Z of the source of space current increases with decreasingfrequency, the low-frequency reaction increases. The normalamplification of the receiver, as shown by curve k0, is, however,simultaneously reduced and the resultant amplification curve is asindicated in Fig. 4 by the curve In. Within a certain frequency rangef1-f2 the amplification approaches infinity; that is, if there is acomponent of feedback voltage ez of this frequency which is in phasewith the vector 61, the receiver goes into selfoscillation. On the otherhand, if there is a component of this voltage in phase opposition to thevector e1, amplitude distortion will result, Therefore, in accordancewith this invention, the design of the receiver is modified so that, atthis frequency, the feed-back voltage ez is 90 degrees out of phase withthe voltage 21 impressed on amplifier l5 and undesirable operatingcharacteristics are avoided, the amplifier having approximately itsnormal characteristic 700.

In Fig. 5 curve 51 shows the phase displacement of the signal caused bythe combined effect of the coupling circuits comprising condenser l9 andresistor 20 and condenser 2| and resistor 22 as a function of frequency.Curve ,62 shows the displacement of the feed-back voltage in the spacecurrent filter 26-2l--28, or its equivalent comprising elements 32 and33 of Fig. 2 Elements i9, 26 and 2t, 22 are usually similarly designedand the total phase displacement of these elements is thus twice thatthrough either one of them. At very low frequencies their combineddisplacement approaches degrees as shown by curve 51. placement of thefeed-back voltage eifected by the space current filter is at highfrequencies and approaches 90 degrees as shown by the curve 62. It wasassumed in the description with respect to Fig. 4 that the instabilityof the kind mentioned occurs between the frequencies of f1 and f2,which'freqencies are also indicated in Fig. 5. The geometric mean valueof frequencies f1, and i2 is represented by in. In accordance with thepresent invention, the receiver constants are so designed that thediiference in the phase displacement of the signal voltage effected bycoupling elements i9, 20 and 2!, 22 and that due to the space currentfilter elements is 90 degrees at the frequency in. This proportionmentmay be had by adjusting either or both of the phase angles c1, 52 (byadjusting elements i9, 26, 21, 22, or 32', 33, respectively) to thecorrect values at the critical frequency in. With such a design,instability no longer occurs within the range between f1 and is. Forfrequencies lower than f1, instability will not occur due to the reducedamplification as shown by curve 760, and for frequencies above f2,instability will not occur due to the low value of impedance Z at thesefrequencies.

In many amplifiers and other apparatus, it is not convenient to alterthe elements included in the amplifier in order to obtain the desiredphase displacement and, consequently, complete stability. In such casesan auxiliary filter of proper design may be-placed between the tube an:-odes and the source of space current, designed as hereinbeforedescribed.

While the description of the invention has; been given in a preferredembodiment utilizing a resistance-coupled amplifier fed from analternating current supply, the invention is equally applicable in otherlow-frequency amplifiers for direct current as well as for alternatingcurrent. In case of transformer coupling in the amplifier, for instance,the phase displacement of the transformer itself may be utilized and thedirect current supply source may be bypassed by means of resistors andcondensers in 75 order to obtain the desired phase displacement Withinthe critical frequency range.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. In a signal-translating system comprising a high-frequencysignal-translating circuit including at least one vacuum tube, a low-frequency amplifier comprising at least one vacuum tube and having an inputcircuit adapted to be coupled to said high-frequency signaltranslatingcircuit, a common source of space current for said tubes, said sourcehaving a substantial impedance within the range of said lowfrequencyamplifier across which a feed-back voltage is developed, and thereactive constants of said amplifier and said source being soproportioned that said feed-back voltage is displaced approximately 90degrees with respect to the input voltage to said amplifier at thefrequency at which said feed-back voltage is greatest.

2. In a signal-translating system comprising a high-frequencysignal-translating circuit including at least one vacuum tube, alow-frequency amplifier comprising at least one vacuum tube and havingan input circuit adapted to be coupled to said high-frequencysignaltranslating circuit, a common source of space current for saidtubes, a filter network coupled between said source and said tubes, saidfilter having a substantial impedance within the range of saidlow-frequency amplifier across which a feed-back voltage is developed,and the reactive constants of said receiver and said filter being soproportioned that said feed-back voltage is displaced approximately 90degrees with respect to the input voltage to said amplifier at thefrequency at which said feedback voltage is greatest.

3. An audio-frequency amplifier for a modulated-carrier signal receivercomprising, a plurality of coupled vacuum tubes, a common source ofspace current for the tubes of said receiver, a filter network coupledbetween said source and the tubes of said receiver, said filter having asubstantial impedance within the audio-frequency range of said amplifieracross which a signal feedback voltage is developed, and the reactiveconstants of said filter being so proportioned that said signalfeed-back voltage is displaced approximately 90 degrees with respect tothe signalinput voltage to the first tube of said amplifier at thefrequency at which said signal feed-back voltage is greatest.

4. An audio-frequency amplifier for a modulated-carrier signal receivercomprising, a plurality of vacuum tubes, impedance means coupling saidVacuum tubes having a phase-shift characteristic variable withfrequency, a common source of anode supply for the tubes of saidreceiver, a filter coupled between said source and the tubes of saidreceiver, said filter having a substantial impedance within theaudio-frequency range of said amplifier across which a signal feed-backvoltage is developed and having a phase-shift characteristic varyingwith frequency oppositely to that of said coupling means, and thereactive constants of said filter and said coupling means being soproportioned that the difference in said phase shifts is approximately90 degrees at the frequency at which said signal feed-back voltage isgreatest.

5. An audio-frequency amplifier for a modulated-carrier signal receivercomprising, a first vacuum tube having input and output electrodes, aresistor-condenser network coupled to the input electrodes of said tube,a second vacuum tube having input and output electrodes, aresistorcondenser network coupling the input electrodes of said secondtube to the output electrodes of said first tube, a common source ofanode supply for the tubes of said receiver, a filter comprising seriesinductance and shunt capacitance coupled between said source and thetubes of said receiver, said filter having a substantial impedancewithin the audio-frequency range of said amplifier across which a signalfeed-back voltage is developed, and the reactive constants of saidcoupling means and said filter being so proportioned that said signalfeed-back voltage is displaced approximately 90 degrees from the inputvoltage to said first tube at the frequency at which said signalfeed-back voltage is greatest.

6. An audio-frequency amplifier for a modu- U lated-carrier signalreceiver comprising, a first vacuum tube having input and outputelectrodes, a resistor-condenser network coupled to the input electrodesof said tube, a second vacuum tube having input and output electrodes, aresistorcondenser network coupling the input electrodes of said secondtube to the output electrodes of said first tube, saidresistor-condenser networks having a combined frequency-dependent phaseshift approaching 180 degrees as a maximum at 1 low frequencies, acommon source of anode supply for the tubes of said receiver, a filtercomprising series inductance and shunt capacitance coupled between saidsource and the tubes of said receiver, said filter having a substantialimpedance within the audio range of said amplifier across which a signalfeed-back voltage is developed and having a phase shift dependent onfrequency and approaching a maximum of 90 degrees at high frequencies,the reactive constants ,1

of said networks and said filter being so proportioned that thedifference in said phase shifts is approximately 90 degrees at thefrequency at which said signal feed-back voltage is greatest.

7. An audio-frequency amplifier for modulatedcarrier signal receiverscomprising, a plurality of coupled vacuum tubes, a common source ofspace current supply for the tubes of said receiver, said supply sourcehaving a substantial impedance within the audio-frequency range of saidamplifier across which a signal feed-back voltage is developed, saidimpedance being a minimum at high frequencies and the amplification ofsaid amplifier being a minimum at low frequencies, whereby said signalfeed-back voltage is a maximum at an intermediate frequency, and thereactive constants of said amplifier and said source being soproportioned that said signal feed-back voltage is displacedapproximately 90 degrees from the signal input to said amplifier at saidintermediate frequency.

CARL-ERIK GRANQV'IST,

